1. Field of the Invention
In one of its aspects, the present invention relates to a fluid treatment system, more particularly an ultraviolet (UV) water treatment system in an open channel, the performance of which is improved by mitigating flow through the low intensity region of the reactor above the top lamp when the system is operated at high fluid velocities. In another aspect of the invention, this is achieved by designing elevation differences between the upstream and downstream ends of the system while obviating or mitigating top lamp overheating due to exposure during periods of low flow.
2. Description of the Prior Art
UV disinfection systems are used in wastewater and drinking water plants to control microbial growth. UV of a wavelength which can be absorbed by cellular nucleotides causes cross-linking, or dimerization, of RNA and DNA, preventing the micro-organisms from replicating and effectively disinfecting the water (Jagger, J. Introduction to research in ultraviolet photobiology. Englewood Cliffs, N.J.: Prentice-Hall Inc., 1967; p.69-73). This technology has been successfully applied commercially in large scale systems since 1981 (U.S. Pat. No. 4,482,809).
Open channels are commonly used to transport or convey large volumes of fluid by gravity and are typically made of concrete. Gravity is used to avoid the high pumping costs that would be incurred if the transport channels were closed and pressurized. Routine cleaning maintenance is made easier by having access to the channel through the open top. Thus, in some applications, it is advantageous when designing a UV disinfection system to make use of conventional open fluid channels to minimize reactor costs and provide for ease of maintenance while still maintaining a gravity flow regime to avoid incurring pumping cost.
Open channel UV disinfection systems for water treatment typically consist of a number of modular assemblies (also known as radiation source modules) arranged in parallel across the width of the fluid canal (U.S. Pat. No. 4,482,809, copending U.S. patent application Ser. No. 09/185,813 (filed Nov. 3, 1998) and copending U.S. patent application Ser. No. 09/258,143 (filed Feb. 26, 1999)). An example of such an assembly is shown in FIG. 1. These assemblies may consist of a number of radiation source modules (1), such as UV lamps enclosed in quartz sleeves, that are substantially parallel to the direction of fluid flow. The modules may be lifted from the channel for routine maintenance, such as sleeve cleaning and lamp replacement. In some cases, an automatic cleaning mechanism is present, consisting of a wiper canister (2) and a drive mechanism (3). The fluid flows past the modules in a relatively unrestricted fashion under the influence of gravity, with a minor difference in water level between the upstream and downstream ends required to allow the water to pass the submerged geometry. This water level difference is dependent on a number of factors, including the complexity of the submerged geometry, the horizontal spacing between modules, the fluid velocity and the like.
The effectiveness of an open channel UV system depends on its ability to deliver the dose of UV required to reach the target disinfection level to each fluid element. The UV dose is defined as the product of UV intensity and reactor retention time, and is normally expressed in mWs/cm2. The inactivation of micro-organisms follows a first order kinetic expression proportional to the UV dose (Jagger, J. Introduction to research in ultraviolet photobiology. Englewood Cliffs, N.J.: Prentice-Hall Inc., 1967; p.69-73). Therefore, the delivery of a uniform dose to all micro-organisms as they pass through the reactor improves reactor efficiency.
As UV passes through a fluid, its intensity at a given point depends on the fluid transmittance of the wavelength in question. The most common germicidal wavelength used is 254 nm, since this wavelength is produced effectively by conventional low pressure mercury arc lamps. Wastewater often exhibits low transmittance of germicidal wavelengths, resulting in a decrease in UV intensity as light travels away from the lamps. This leads to regions of lowest UV intensity between lamps. A certain spacing, dependent on water quality and lamp type is desirable to ensure adequate dose delivery and reactor performance.
The residence time required for adequate dose delivery depends on UV intensity. For poor water quality, UV transmittance and average UV intensity are low, usually requiring relatively long residence times within the reactor to maintain adequate dose. This is typically accomplished by designing the UV disinfection system with banks of UV modules disposed serially. Conversely, with good water quality or high intensity radiation sources, residence time is normally decreased to achieve the same dose. This is usually accomplished by designing relatively narrow fluid channels, decreasing the amount of UV equipment required as compared with a system installed in a wide channel. This results in relatively high fluid velocities past the modules, leading to significant differences in water level between the upstream and downstream ends of the system.
Since UV radiation from each lamp only penetrates a certain distance into the fluid, each lamp can effectively treat a layer of water around it, usually equal to approximately one half the spacing between lamps. When large water level differences cause the layer of water above the top lamp to exceed this critical value, the excess water travels through a zone of relatively low UV intensity, exiting the irradiation zone with lower than adequate dose. This fluid xe2x80x9cshort circuitxe2x80x9d leads to non-uniform dose distribution, resulting in micro-organisms escaping essentially untreated, leading to poor overall reactor performance.
It is desirable to have a reactor which obviates or mitigates water level differences that lead to a xe2x80x9cshort circuitxe2x80x9d above the top lamp over a relatively wide range of water qualities and flow rates. The submerged geometry is fixed by the design of the modular assemblies and usually cannot easily be modified without comprising equipment functionality. Horizontal spacing often cannot be increased without producing regions of low UV intensity in the reactor. For good water quality or high intensity radiation sources, it is therefore usually important to operate a UV disinfection system at high fluid velocities for the reasons described above.
It would be desirable to have a fluid treatment system capable of obviating or mitigating a xe2x80x9cshort circuitxe2x80x9d effect over the top lamp of a UV disinfection system when operated at high fluid velocities using elevation differences between the upstream and downstream ends of the reactor.
It is an object of the invention to provide a novel fluid treatment system which obviates or mitigates at least one of the above-mentioned disadvantages of the prior art.
Accordingly, in one of its aspects, the present invention provides a fluid treatment system comprising at least one radiation source disposed in an open irradiation zone, the irradiation zone have a fluid inlet which is elevated with respect to a fluid outlet.
In another of its aspects, the present invention provides a fluid treatment system comprising an array of radiation sources disposed in an open channel, an upstream end of the open channel adjacent an upstream end of the array being elevated with respect to a downstream end of the open channel adjacent a downstream end of the array.
In yet another of its aspects, the present invention provides a fluid treatment system comprising a first irradiation zone and a second irradiation zone disposed at different heights with respect to one another such that the fluid level is substantially normalized with respect to the top of each of the first irradiation zone and the second irradiation zone.
In yet another of its aspects, the present invention provides a fluid treatment system comprising a plurality of serially disposed irradiation zones which are elevated with respect to one another thereby substantially normalizing the fluid level with respect to the top of each irradiation zone.
In yet another of its aspects, the present invention provides a fluid treatment system comprising a first irradiation zone and a second irradiation zone disposed at different heights with respect to one another such that the fluid level is substantially normalized with respect to the top of each of the first irradiation zone and the second irradiation zone.
In yet another of its aspects, the present invention provides a fluid treatment system comprising a canal for receiving a fluid to be treated, a first upstream irradiation zone and a second downstream irradiation zone which is stepped with respect to the first upstream irradiation zone.
A UV disinfection system, consisting of multiple banks of modular assemblies in hydraulic series, may be constructed using different elevations for each bank. This can be accomplished using a step for the upstream bank and/or by placing both banks on a slope. Water level differences resulting from fluid flow past submersed bodies (xe2x80x9chead lossxe2x80x9d) propagate upstream from a fixed level point, such as a gate, weir or the like.
In one embodiment, a step elevates an upstream bank relative to a downstream bank to prevent an excessively high water level above the top lamp of the upstream bank. Preferably, the step comprises a height in the range of from about 0.5 to about 7.0 inches, more preferably in the range of from about 0.5 to about 5.0 inches, even more preferably in the range of from about 1.0 to about 4.0 inches, most preferably in the range of from about 1.0 to about 3.0 inches.
A slope can also be used to provide elevation of the upstream bank, so that little or no change in water level is desirable to cause fluid to flow. Preferably, the sloped surface comprises a slope of at least about 0.2%, more preferably in the range of from about 0.2% to about 6.0%, even more preferably in the range of from about 0.2% to about 3.0%, even more preferably in the range of from about 0.5% to about 2.5%, most preferably in the range of from about 1.0% to about 2.0%. As those of skill in the art will recognize, the xe2x80x9cslopexe2x80x9d be readily determined by dividing the rise of the sloped surface by the run of the sloped surface.
The size of step or angle of slope may be selected by the combination of design velocity and submerged geometry for a given treatment system. As velocity increases or submerged geometry becomes more restrictive, the potential energy required for fluid flow increases, mandating a larger step height or angle of slope. When flow rate drops below the design value, the top lamps of the elevated bank can become exposed, resulting in high lamp temperatures and premature lamp failures. Cooling of the affected lamps can be accomplished by directing a fluid, such as water or air, to flow over the lamps, thereby removing heat and reducing lamp temperature. Alternatively, the affected lamps may be switched off to reduce energy consumption and prevent premature high temperature failure from occurring.
Preferably, the term xe2x80x9cfluid inletxe2x80x9d, when used in the context of the irradiation zone, means any plane located upstream of the radiation source(s) and through which fluid may flow. Further, the term xe2x80x9cfluid outletxe2x80x9d, when used in the context of the irradiation zone, means any plane located downstream of the radiation source(s) and through which fluid may flow.